1. Field of the Invention
The present invention relates to the field of fabrication of semiconductor devices. More specifically, the invention relates to the fabrication of silicon-germanium semiconductor devices.
2. Related Art
In a heterojunction bipolar transistor, or HBT, a thin silicon-germanium layer is grown as the base of a bipolar transistor on a silicon wafer. The silicon-germanium HBT has significant advantages in speed, frequency response, and gain when compared to a conventional silicon bipolar transistor. Speed and frequency response can be compared by the cutoff frequency which, simply stated, is the frequency where the gain of a transistor is drastically reduced. Cutoff frequencies in excess of 100 GHz have been achieved for the HBT, which are comparable to the more expensive GaAs. Previously, silicon-only devices have not been competitive for use where very high speed and frequency response are required.
The higher gain, speeds, and frequency response of the HBT have been achieved as a result of certain advantages of silicon-germanium not available with pure silicon, for example, narrower band gap, and reduced resistivity. Silicon-germanium may be epitaxially grown on silicon wafers using conventional silicon processing and tools, and allows one to engineer device properties such as the band gap, energy band structure, and mobilities. For example, it is known in the art that grading the concentration of germanium in the silicon-germanium base builds into the HBT device an electric field, which accelerates the carriers across the base, thereby increasing the speed of the HBT device compared to a silicon-only device. One method for fabricating silicon and silicon-germanium devices is by chemical vapor deposition (xe2x80x9cCVDxe2x80x9d). A reduced pressure chemical vapor deposition technique, or RPCVD, used to fabricate the HBT device allows for a controlled grading of germanium concentration across the base layer. As already noted, speeds in the range of approximately 100 GHz have been demonstrated for silicon-germanium devices, such as the HBT.
Epitaxial growth on a silicon surface of silicon-germanium that is sufficiently high quality to meet the demands of fabrication for devices such as the HBT demands that the silicon surface be as near as possible to a perfect crystal surface. Specifically, the silicon surface must not be contaminated. The presence of contaminants on the silicon surface cause defects to occur in the subsequent silicon-germanium crystal growth on top of that surface. For example, contaminants on the silicon surface at the start of epitaxial deposition can result in accelerated growth, referred to as a xe2x80x9cspike.xe2x80x9d The height of a spike can be as great as the thickness of the epitaxial deposition. Such a spike can cause holes in photoresist and other subsequently deposited layers of material.
Because silicon-germanium integrates two dissimilar materials, defects compromise the crystalline quality regardless of the amount of germanium present. Defects cause even greater compromise in crystalline quality as the concentration of germanium increases in the silicon-germanium. Therefore, demands that the silicon surface be as near as possible to a perfect crystal surface become even stricter for silicon-germanium than for silicon-only devices.
The presence of contaminants on the silicon surface can adversely affect yield of the fabrication process, device performance, and device reliability. For example, defects due to contamination may cause the wafer to fail quality checks and inspections within the fabrication process resulting in fewer wafers completing the fabrication process and higher costs due to lower yield. Device performance can be changed, for example, by unwanted mobile ionic contaminants resulting in a device unsuitable for the use for which it was designed. Device reliability can be adversely affected, for example, by small amounts of metallic contaminants which can travel in the device and eventually cause failure. Therefore, it is important to control the presence of contaminants on the silicon surface in order to prevent adverse effects on yield, performance, and reliability of silicon-germanium devices.
Contaminants, which must be removed from the silicon surface, include particulate matter, organic residue, and inorganic residue. By way of example, particulate matter includes dust and smoke particles, as well as other impurities commonly found in the air, and bacteria that grow in water systems and on surfaces not cleaned regularly. Organic residues are chemical compounds containing carbon; for example, oils in fingerprints. Inorganic residues are chemical compounds not containing carbon; for example, hydrochloric acid or hydrofluoric acid which may be introduced from other steps in the wafer processing. As these examples indicate, the sources of contamination include materials which are omnipresent in the environment, such as carbon and oxygen, but also include other steps in the fabrication process, for example, chemical residue on RPCVD chamber walls or residual oxides from typical cleaning solvents such as peroxides.
One method for cleaning the wafer surface prior to epitaxial deposition processes is to employ a sequence of heated, peroxide-charged hydrochloric acid and ammonia hydroxide baths. Very harsh solvents can be used because the silicon surface is extremely resistant to almost all acids and bases. The silicon surface, however, will almost immediately react with and bind to impurities that are always present in the air and in aqueous solutions. By way of contrast, an oxygenated silicon surface (i.e. glass) is quite inert. Oxygen is therefore provided in the final step of the clean in order to form a glassy silicon oxide protective surface over the silicon surface. The silicon oxide protects the previously exposed silicon surface while the wafer is in transition from the cleaning area to the RPCVD chamber.
However, prior to subsequent epitaxial deposition, the protective silicon oxide and any residual silicon oxide must be removed from the silicon surface. One method, referred to as a xe2x80x9chydrogen bake,xe2x80x9d is that the protective silicon oxide and any residual silicon oxide can be evaporated from the silicon surface in the RPCVD chamber by heating the wafer to a temperature in a range of approximately 700xc2x0 C. to 1100xc2x0 C. for a period of approximately 5 to 30 minutes in a hydrogen atmosphere. The hydrogen desorbs residual contaminants present on the surface of the wafer during the hydrogen bake. The purpose of the hydrogen bake is to leave the silicon surface in the epitaxial deposition region completely free of contaminants.
One problem of the high temperature hydrogen bake arises from the fact that it permits opportunities for recontamination due to contaminants that migrate from other areas on the wafer to the silicon surface in the epitaxial deposition region. For example, contaminants can migrate at the elevated temperatures during the hydrogen bake from the isolation regions onto the epitaxial growth region.
Recontamination can result, for example, from contamination present from prior processing steps. Sources include residual contaminants on the RPCVD chamber walls or on the wafer itself or out-diffusion of dopants from the silicon substrate. Examples of contaminants include oxygen, water, and organo-metallic compounds that can contaminate oxides on the surface of the wafer such as isolation regions. Recontamination adversely affects the yield, performance, and reliability of silicon-germanium devices as discussed above.
Thus, there is need in the art to improve the yield for fabrication of silicon-germanium semiconductor devices, as well as device performance and reliability, by improving the methods used to control the presence of contaminants on the silicon surface prior to epitaxial growth of silicon-germanium.
The present invention is directed to method for reducing contamination prior to epitaxial growth and related structure. The invention overcomes the need in the art to improve the yield for fabrication of silicon-germanium semiconductor devices, as well as device performance and reliability, by improving the methods used to control the presence of contaminants on the silicon surface prior to epitaxial growth of silicon-germanium.
According to the invention the surface of a semiconductor wafer is covered by an etch stop layer. For example, the etch stop layer can be composed of silicon dioxide. A cap layer is then fabricated over the etch stop layer. For example, the cap layer can be a polycrystalline silicon layer fabricated over the etch stop layer. The cap layer is then selectively etched down to the etch stop layer creating an opening in the cap layer according to a pattern. The pattern can be formed, for example, by covering the cap layer with photoresist and selective etching. Selective etching can be accomplished by using a dry etch process which etches the cap layer without substantially etching the etch stop layer. The etch stop layer is then removed using, for example, a hydrogen-fluoride cleaning process. A semiconductor crystal is then grown by epitaxial deposition in the opening. For example, the semiconductor crystal can be silicon-germanium.
Moreover, a single crystal semiconductor structure of high quality, i.e. with virtually no crystal defects, can be formed according to one embodiment of the invention. For example, according to one embodiment, a high quality silicon-germanium base region of a heterojunction bipolar transistor can be formed on a silicon wafer.